The present invention relates in general to SO.sub.2 scrubbing, and in particular to a new and useful method that takes into account recycled flyash.
Dry SO.sub.2, scrubbing is a process where a mixture of water, lime, and flyash is injected and mixed with a hot flue gas stream to reduce the SO.sub.2 content in the flue gas stream. As the SO.sub.2 scrubbing process proceeds, the temperature of the flue gas decreases due to the quenching effect of the water. Also, any by-product for the process (e.g., reacted lime, flyash, etc.) will exit the system as a dry material.
The mixture of water, lime, and flyash is injected into the flue gas stream as a slurry that is atomized to small particles by a dual-fluid atomizer. The atomized slurry is then mixed with the flue gas by the increased turbulence and swirling motion introduced into the flue gas by a specialized nozzle structure known by the trademark Turbo-Diffuser.
To increase the SO.sub.2 absorption in the dry scrubber while minimizing lime consumption, it is necessary to recycle reactive flyash from the system through the atomizers. The method involves preparation of a flyash slurry, subsequent mixing with a lime slurry and then the pumping and transport to and through the atomizers as described in U.S. Pat. No. 4,452,765 which is hereby incorporated by reference.
A technical paper, "Comparison of Dry Scrubbing Operation of Laramie River and Craig Stations" presented by J. B. Doyle, B. J. Jankura, and R. C. Vetterick to the Symposium on Flue Gas Desulfurization in Atlanta, Ga, Nov. 16-21, 1986, discusses the advantage of using high-alkali ash, along with the problems caused when using this ash. A technical paper, "Dry SO.sub.2 System Design and Early Operating Experience at Basin Electric's Laramie River Station", presented by G. L. Anderson, W. DePriest, and E. I. Edwards to the ASME Joint Power Generation Conference in Denver, Co, Oct. 17-21, 1982 discloses that the key to an effective dry sulfur removal process is intimate gas/slurry contact. This reference teaches using three independent alkali preparation processes: a lime slurry preparation, a recycle ash slurry preparation, and water conditioning.
U.S. Pat. No. 4,645,652 discloses the use of a dry alkaline reactant, which includes flyash, pneumatically injected into a duct.
None of the references disclose pneumatically injected recycled flyash supplied simultaneously with humidification into a highly turbulent flow region at the exit of the atomizer for assisting in mixing the air/flyash mixture with the lime slurry spray.